text: You have written incorrect code.
  
  1) spWeb.Files.Add
  
  2) use using block for SPWeb in above case.
  
  Please go through it you will be able to resolve the issue.
  
  using (SPSite site = newSPSite("url"))
    {
  
  using (SPWeb web = site.OpenWeb())
    {
  
  SPSecurity.RunWithElevatedPrivileges(delegate()
                 {
  
  SPDocumentLibrary documentLib = web.Lists["YourLibraryName"] as SPDocumentLibrary;
  
  Stream fStream = FileUpload1.PostedFile.InputStream;
  byte[]  _byteArray = new byte[fStream.Length];
  fStream.Read(_byteArray, 0, (int)fStream.Length);
  fStream.Close();
  
  web.AllowUnsafeUpdates = true;
  
  string _fileUrl = documentLib.RootFolder.Url + "/" + FileUpload1.PostedFile.FileName;
  
  bool IsOverwriteFile = true;
  SPFile file = documentLib.RootFolder.Files.Add(_fileUrl, _byteArray, IsOverwriteFile);
  
  SPListItem item = file.Item;
  item["Title"] = FileUpload1.PostedFile.FileName;
  item.Update();
  file.Update();
  
  web.AllowUnsafeUpdates = false;
  
        });           
   }
  
   }